Hitherto, there have been known various packages for cooking in a microwave oven, in which retort food, frozen food, or the like is contained in synthetic resin containers to be sealed, and heated in the microwave oven to cook the contained products at the time of eating. However, when those packages are heated in the microwave oven, inside pressure of the packages is increased by steam and the like generated from the food contained therein, the packages explode, and the food may be scattered. As a result, the inside of the microwave oven becomes a mess, and harm such as a scald may be inflicted on the human body.
Therefore, before the above-mentioned package is heated for cooking in the microwave oven, the package is partially unsealed or a hole is produced in the package to discharge steam and the like, which are generated in the package, to an outside thereof so as to prevent explosion of the package.
However, the above-mentioned method requires care to an ordinary consumer. Because steam generated from the food inside the package by heating in the microwave oven is discharged to the outside of the package immediately, the package has a disadvantage in that the effect of heating and steaming the food by steam lowers, and the taste of the food become worse.
In order to solve the above-mentioned problem, various packages for cooking in a microwave oven have been proposed, in which when a container made of a synthetic resin and having a flange part is filled with food and a lid is heat-sealed to the flange part, a projection part that projects in an inward direction of the container is formed at a heat-sealed part. Those packages are prevented from exploding by allowing the projection part to be automatically opened due to the increase in inside pressure in the container in heating/cooking in a microwave oven. (For example, see Patent Documents 1-3)    Patent Document 1: JP Sho 62-235080 A    Patent Document 2: JP Hei 11-171261 A    Patent Document 3: JP 2000-62858 A
However, in those packages, when the container is filled with contents such as rice, and heat-sealed to form the projection part that automatically opens, a sealing tool may be displaced from a predetermined part of the container, with the result that the projection part may be displaced from the predetermined position. Consequently, the stress generated inside the container is unlikely to be concentrated at the projection part in heating/cooking in a microwave oven, and a sealed part other than the projection part may open. Further, the package is opened earlier than a predetermined time, with the result that an effect of improving a taste of the contents through a steaming effect decreases.
Further, because the lid adheres to the container body immediately after the container automatically opens and steam is released in heating/cooking in a microwave oven, it becomes difficult to take outside air into the container rapidly. Consequently, the package is deformed Largely, resulting in being difficult to take out the contents cooked by heating.
Further, in the case where the contents such as rice adhere to the heat-sealed part between the lid and the flange part at a time of filling of the contents, it is difficult to remove the adhered contents in a conventional package due to a flat periphery of the heat-sealed part. Consequently, the heat-sealed part peels immediately after the filling of the contents or at a time of distribution of the package, whereby leaking out of the contents or the like may occur.